1. Field of the Invention
This invention relates to a motor controller for driving a motor whose rotation range is set and, more specifically, to a motor controller for use in an electric power steering controller which helps control a steering wheel, for example.
2. Description of the Prior Art
Motor controllers for driving a motor whose rotation range is set include an electric power steering controller which helps control a steering wheel, for example. In this electric power steering controller, the amount of motor rotation is determined by the rotation degree of a steering wheel.
Heretofore, fault detection means for use in such a controller as disclosed in Japanese Patent Publication No. 4-31171 has been proposed.
FIG. 21 shows the substantial configuration of an electric power steering controller of the prior art. This electric power steering controller comprises a motor 6 for assisting steering force, a clutch 7 interposed between the motor 6 and a steering system, a torque sensor 4 for detecting the input torque of the steering shaft, and a control unit 1 connected to a speed sensor 5 for detecting the speed of a vehicle. The control unit 1 is also connected to a battery (main power source) 2 and a key switch 3.
The control unit 1 comprises a motor drive circuit 14 having a bridge circuit 34 which consists of power transistors 30, 31, 32, 33 and a current detecting resistor 35 on the current flow-out side of the bridge circuit 34, and the transistors 30, 31, 32, 33 constituting the bridge circuit 34 are controlled by a CPU 10. The current detecting resistor 35 is connected to the CPU 10 through motor current detection means 15. A clutch drive circuit 16 for driving the clutch 7 is also connected to the CPU 10. One terminal of the motor 6 is connected to one input terminal of a differential amplifier 22 and the other terminal of the motor 6 is connected to the other input terminal of the differential amplifier 22 through an offset circuit 23. The output terminal of the differential amplifier 22 is connected to the CPU 10 so that a voltage at both terminals of the motor 6 is supplied to the CPU from the differential amplifier 22 as a positive or negative value with respect to an offset amount according to the rotation direction of the motor 6.
In the above-described electric power steering controller, the motor drive circuit 14 compares a motor application voltage to be applied to the motor 6 with a voltage at both terminals of the motor supplied from the differential amplifier 22 in response to a control signal from the CPU 10 so as to detect a fault in the motor drive circuit 1 based on the difference between them. When a fault is detected, fail-safe processing is performed to turn off the relay contact of an unshown fail-safe relay circuit. With such a configuration, it is possible to detect a fault in wiring as well as a fault in the elements of the motor drive circuit 14.
However, in an electric power steering controller comprising the motor drive circuit 14 shown in FIG. 21 in which a motor current is detected by the current detection resistor 35 provided on the current flow-out side of the bridge circuit 34, when the power transistor 30 of the motor drive circuit 14 is PWM driven, the power transistor 33 is fixed at an "ON" position, and the power transistors 31 and 32 are turned off to drive the motor 6, it is difficult to detect a fault because a detected voltage value between the motor terminals changes little even if the motor terminal connected between the power transistors 32 and 33 is grounded. Further, since a circuit for detecting a voltage between motor terminals requires high detection accuracy, precision parts are used to enhance detection accuracy with the result of boosted costs. Moreover, when a motor terminal is grounded having a contact resistance under the above condition, a motor current smaller than a current running through the motor 6 is detected since the motor current is split by the current detection resistor 35 and the above-described contact resistance. As the result, over-assistance is provided by the motor 6, and hence, the steering wheel becomes lighter.
If the current detection resistor 35, not shown, is provided on the current flow-in side of the bridge circuit 34, when the power transistor 30 of the motor drive circuit 14 is PWM driven, the power transistor 33 is fixed at an "ON" position, and the power transistors 31 and 32 are turned off to drive the motor 6, the steering wheel becomes lighter as well even if the motor terminal connected between the power transistors 30 and 31 is short-circuited to the positive side of a power source (referred to as "short-circuited" hereinafter).